Almost all the alum produced today is manufactured from bauxite or bauxitic or other clays which are found primarily in Ala., Ark., Ga., Mo., Jamaica, Guyana, Surinam, other South American countries and in Asia. Alumina-bearing ores are usually mined by open pit methods, crushed, calcined, ground and shipped to the alum manufacturing plant. At the plant, the ore is charged, together with the required amount of sulfuric acid, into digesters, where, after several hours at elevated temperature, the reaction is completed as represented by the following equation: EQU Al.sub.2 O.sub.3 +3H.sub.2 SO.sub.4 +11H.sub.2 O.fwdarw.Al.sub.2 (SO.sub.4).sub.3 +14H.sub.2 O
One of the most important uses for alum is in the manufacture of paper. Alum is used to improve drainage, retain fines, set rosin and control pH and also to enhance the value of resin additives. Alum is also used in the paper-making process to control pitch, prevent foam and improve the properties of broken and secondary fibers. Proper application and control of the alum level employed increases machine production and can reduce the input of many other chemical additives.
Alum solutions, made according to the method discussed above, almost always contain varying amounts of iron impurities, usually depending upon the type and origin of the ore. These iron impurities, when present at high levels, impart undesirable discoloration to paper when used in the preparation thereof and, in addition, they can catalyze the decomposition of bleaching agents used in the production of white paper stock. Several patents have issued which are directed to the production of iron-free alum (aluminum sulfate), see U.S Pat. No. 4,039,615 and 4,244,928, however, none of these procedures have proven commercially suitable for the manufacture of technical grade alum for use in paper-making processes.
The process of the present invention is designed to more effectively remove iron from concentrated alum solutions. The improvement forming the basis of the present invention lies in adding and efficiently mixing a polymer containing hydroxamic acid groups into the concentrated alum solution, preferably after having subjected the solution to oxidation, either chemically or through aeration, to convert the Fe.sup.2+ iron to the Fe.sup.3+ species, thus forming an insoluble polymer-iron complex which can be separated from the alum by decantation. This treatment leads to reduced levels of iron in the concentrated alum solutions when compared with state-of-the-art processes, as exemplified by the patents mentioned above. Such reductions in iron enable the alum solution to be utilized in the manufacture of paper, especially white stock, free of the undesirable discoloration normally attendant the presence of iron in said solutions.
Hydroxamated polymers are well known to these skilled in the art and are specifically disclosed, as are methods for their production, in Japanese Pat. No. 18,865/65; European Patent Application 104,970; U.K. Patent Application 2171127 and U.S. Pat. Nos. 3,345,344; 4,480,067; 4,532,046; 4,536,296 and 4,587,306, hereby incorporated herein by reference.
These polymers are known to be chelating agents for iron, as well as many other polyvalent metal ions, as disclosed in various of those U.S. patents set forth above, as well as articles by Kern et al, Synthetic Macromolecular Substances with Reactive Groups; Angew, Chem. 69; 153-171; 1957; and Winston et al; Polym. News; 10; 6-12; 1984; and Jour of Polymer Sci.; 14; 2155-2165; 1976; and 2131-2143; 1984; and Macromolecules; 1978; 11; 597-603 and 1981; 14; 538-543 and Polymer News; 10 6-12, 1984. Monomeric hydroxamic acids are taught as extractants in U.S. Pat. Nos. 3,821,351; 3,900,551 and 4,474,742. None of these references, however, teach the removal of iron from concentrated alum solutions using hydroxamic acid or salt group containing polymers. The fact that the hydroxamated polymers, which can, in principle, bind both iron and aluminum, preferentially remove low concentrations of iron from essentially saturated, strongly acidic solutions of aluminum is an unexpected finding.